The field of precision electronic distance measurement has long been dominated by microwave type systems. These systems are, however, normally limited to line-of-sight operation. Usually this is not a problem in terrestrial operations, but for offshore uses it is an important limitation. Offshore, over-the-horizon electronic surveying is an application that can be suited to a system employing ground wave propagation. Several ground wave systems are presently used for navigation; these include Loran A, Loran C, Decca, Omega, Consolan and DF. However, none of these systems, except possibly Loran C, is sufficiently accurate for surveying. For the application of surveying, certain specialized systems including Toran, Raydist and Lorac have been developed. These systems are however, susceptible to sky waves and have difficulties with ambiguity resolution. In addition, their accuracy has been relatively insufficient for certain applications, notably offshore petroleum surveying. Further such systems often require a rather large frequency bandwidth that is often unavailable, and the systems lack flexibility in changing the signal frequency for operation, that is often required in use of the systems. Such systems do not normally permit a "narrow band" operation of two closely spaced frequencies, and thus often require a pre-fixed frequency band for operation that is not available in the particular locality in which the system is to be used.
It is thus advantageous to have a new and improved offshore, over-the-horizon, electronic surveying and ranging system.